Revert 158219 "Move dom/*Event* to events to match the DOM vs. E..."

Revert 158219 "Move dom/*Event* to events to match the DOM vs. E..."

> Move dom/*Event* to events to match the DOM vs. Events spec separation
> 
> This is not quite perfect, but it does make the DOM directory much
> smaller and thus hopefully much easier to reason about.
> 
> R=abarth@chromium.org
> 
> Review URL: https://codereview.chromium.org/23983049

TBR=eseidel@chromium.org